There are two major visual inputs to the central circadian pacemaker of mammals: a direct pathway from the retina to the pacemaker and an indirect pathway which passes through the lateral geniculate nuclei. This research project concerns the indirect visual tract or geniculohypothalamic tract (GHT) which projects to the central clock located in the hypothalamus. The GHT conveys information pertaining to the level of ambient illumination to the central clock. This information is first transmitted through the optic tract to the intergeniculate leaflet (IGL) of the lateral geniculate nucleus. Neuropeptide Y (NPY) cell bodies are located within the IGL. These NPY cell bodies project fibers to the clock located in the suprachiasmatic nucleus of the hypothalamus. Very little is known regarding those factors that regulate NPY synthesis within cell bodies of the IGL. We have begun to investigate the circadian and light regulation of NPY mRNA synthesis within the GHT. Using in situ hybridization histochemical techniques, other investigators have recently demonstrated the presence of mRNA encoding for NPY in rat arcuate nuclei and cortex. Using these techniques last year we identified NPY mRNA in the vicinity of the IGL cells of Syrian hamsters. During the next year we will examine the circadian pattern of NPY mRNA within the IGL with the pattern in hypothalamic nuclei that do not receive a visual projection. In addition, we will investigate the effects of antidepressant drugs on the level of mRNA.